1. Technical Field
This invention relates to pneumatic radial tires of steel-belted type for automotive use. More particularly, the invention relates to such a tire having significant weight saving while providing sufficient driveability performance.
2. Prior Art
In harmony with increasing calls for conserving the global environment, moves have recently been growing in the automotive industry to save the per mileage consumption of fuel oils by motor vehicles. One goal is directed, to cope with that trend, toward reducing the weight of pneumatic radial tires. The fuel amount spent by a vehicle is closely associated with the tire weight.
Steel cords are in common use, for their notably higher tensile strength and elastic modulus properties than fiber cords of plastic materials, as reinforcements for belt layers in radial tires. Lightweight radial tires, however, are not easily available with such a steel cord as it is inherently large in specific gravity. No new class of fibrous cords has become known which, from standpoints of mechanical strength, could take the place of the existing steel cords. That leaves automotive tire manufacturers little alternative but to rely on those steel cords in constructing radial tires.
To produce a weight-saving steel-radial tire, a positive attempt is expected to be made with a focus on reducing the number of steel wires fabricated into a steel cord and hence on holding two certain gauges of rubber to an absolute minimum in the tire geometry. One such gauge is the distance or depth between the bottom of grooves defined in a tread and the surface of steel cords embedded in a double-plied belt in close proximity to the groove bottom, and another is the interlaminar spacing between the steel cords disposed in the upper and lower belts. In that instance, the steel cord may be formed by orienting a single straight steel wire, but not by twisting a plurality of steel wires together into an integral structure as commonly accepted in the art. The single-wired steel cord has a smaller cross-sectional area than the multiple-wired twisted counterpart and hence allows the above gauges to be set at as small a level as possible with consequential saving of the quantity of rubber used in a steel-radial tire. Also advantageously, the steel cord so obtained can be completely covered with a coat rubber and hence protected against corrosive fatigue and adhesive failure even in moist condition.
However, the single-wired steel cord is too rigid, due to the steel wire being straight without twist or warp, to be endurable relative to deformation on bending or compression under repeated load. This means that the steel cord tends to break more frequently than the multiple-wired twisted steel cord, resulting in a belt layer of marred durability.